1. Field of the Invention
The present invention relates to a rotary encoder, and more particularly to a rotary encoder in which a light beam from a laser is irradiated to a rotary scale (rotational scale) having a radial diffraction grating (amplitude type diffraction grating) having a plurality of grating patterns of light transmission areas and light reflection areas periodically arranged along a circumference. The a rotating angle and a velocity of the rotary scale is photo-electrically detected by utilizing a diffracted light from the diffraction grating.
A rotary encoder which uses a rotary scale having a diffraction grating arranged on a periphery of a disk connected to a rotating member and in which a laser beam is irradiated to the diffraction grating to interfere diffracted lights generated by the diffraction grating, and a rotating angle and a rotating speed of the rotary scale (rotational body) are detected by detecting a change in intensity of the interfered light, has been known in the art.
FIGS. 1A and 1B show a structure of a conventional rotary encoder disclosed in U.S. Pat. No. 4,792,678. Numeral 1 denotes a laser, numeral 2 denotes a disk (rotary scale) having a radial diffraction grating formed thereon, numeral 3 denotes a reflection prism, numeral 4 denotes a polarization prism, numerals 51 and 52 denote photo-sensors, and numeral 6 denotes a rotating shaft of the disk 2. In FIG. lA, a laser beam emitted from the laser 1 is directed to a point M.sub.1 on the diffraction grating of the disk 2 essentially normally. A plurality of .+-. 1-order diffraction lights generated at the point M.sub.1 are reflected orthogonally by a first orthogonal reflection plane 3a of the reflection prism 3, totally reflected twice by each of side planes 3c and 3d of the reflection prism 3, again orthogonally reflected by a second orthogonal reflection plane 3b of the reflection prism 3, and directed to a point M.sub.2 on the diffraction grating of the disk 2. FIG. 1B illustrates a light path of the .+-. 1-order diffraction lights in the reflection prism 3. It is a plan view of the reflection prism 3 shown in FIG. 1A, as viewed from the bottom. As shown in FIG. 1B, the .+-. 1-order lights generated at the point M.sub.1 are directed to different directions from each other at diffraction angles .alpha.+and .alpha.-, and totally reflected by the side planes 3c and 3d of the reflection prism 3. They cross near the center of the reflection prism 3, are again totally reflected by the side planes 3c and 3d, and are directed to the points M.sub.2 on the diffraction grating 2 from the different directions at the same angles as the diffraction angles .alpha.+ and .alpha.-. Thus, the .+-. 1-order rediffracted lights at the point M.sub.2 exit from the diffraction grating in superposition and parallel to the incident light from the laser 1 at the point M.sub.1. The superimposed .+-. 1order rediffracted lights are received by the photo-sensors 51 and 52 through the polarization prism 4. The phases of the .+-. 1order diffracted lights change by .+-. 2.pi. as the diffraction grating of the disk 2 rotates by one grating pitch. Similarly, the phases of the .+-. 1order rediffracted lights change by .+-. 4.pi. as the diffraction grating rotates by one grating pitch. Thus, when the .+-. 1-order rediffracted lights are superimposed and interfered with each other as shown in FIGS. 1A and 1B, the photo-sensors 51 and 52 produce four periods of sine wave signals in one grating pitch rotation of the diffraction grating of the disk 2. Thus, the photosensors 51 and 52 produce 4N periods of the sine wave signals in one rotation of the disk 2 where N is the total number of gratings on the diffraction grating. In FIGS. 1A and 1B, the points M.sub.1 and M.sub.2 are substantially symmetric with respect to the rotation center of the rotating shaft 6 so that a measurement error is prevented even if an eccentricity is involved in the mounting of the disk 2 on the rotating shaft 6. The photo-sensors 51 and 52 produce first and second signals having a 90.degree. phase difference therebetween by the combination of the direction of the linear polarization light of the laser beam emitted from the laser 1, the conversion to the .+-. 1-order elliptic polarization lights by the total reflection in the reflection prism 3, and the polarization prism 4. Thus, the rotating direction of the disk 2 may be determined by comparing those signals. In the prior art rotary encoder described above, the following problems are encountered.
(1) The light path of the diffracted light is complex and the assembly and adjustment are difficult to attain. PA1 (2) The light path of the interfering .+-. 1order PA1 (3) Because the reflection prism 3 is on the extension of the rotating shaft 6, it is difficult to adopt a hollow structure which is useful in the rotary encoder.
diffracted lights crosses another light path in the reflection prism 3. As a result, a measurement error may be easily introduced by an environmental change such as surrounding temperature change and temperature distribution. The larger the diameter of the disk 2 is, the longer is the light path of the .+-. 1-order diffracted lights in the reflection prism 3, that is, non-common light path, and hence the error will more likely be introduced.